Advances in the art of coupling between an external optical signal and an optical waveguide have recently been associated with the use of an optical “nanotaper” structure. A “nanotaper”, which is sometimes referred to as an “inverse taper”, is generally defined as a terminating portion of a core of a high index waveguide that is used to facilitate efficient coupling between a single mode optical fiber (for example) and a high index waveguide formed along an optical substrate. In a typical device construction, the lateral dimension of the portion of the nanotaper proximate to the core of the high index waveguide approximately matches the width of the core. The lateral dimension of the nanotaper decreases monotonically along the direction of light propagation until it reaches a small value associated with a “tip” (i.e., that portion of the nanotaper distal from the core of the high index waveguide). The tip portion represents the point at which light first enters or exits the nanotaper.
In some prior art nanotapers, the device is cleaved such that the endface of the tip essentially coincides with a cleaved edge of the optical substrate. Light is then launched directly into the tip of an entry nanotaper (or extracted directly from the tip of an exit nanotaper) by aligning the light source/receiver with the cleaved edge of the substrate. However, the presence of the high index nanotaper tip at the junction where the incoming signal couples into the optical substrate has been found to generate back-reflections, presenting problems when attempting to directly couple light from a laser facet into the waveguide. In fact, the back-reflections may cause the laser to become unstable.
Alternatively, in other prior art nanotapers, the position of the tip is recessed from the cleaved edge of the optical substrate. An auxiliary waveguide is then used to transmit light from the cleaved edge to the tip of the nanotaper. The auxiliary waveguide generally comprises larger dimensions and a lower refractive index than the single mode optical waveguide to improve coupling efficiency. The core of the auxiliary waveguide may comprise a polymer-based material with a refractive index on the order of 1.5-1.6.
One particular prior art nanotaper coupler arrangement using an auxiliary waveguide is shown in FIGS. 1 and 2. The auxiliary waveguide takes the form of a first, larger-dimensioned waveguide section that is generally disposed in an overlap arrangement with respect to a second, smaller-dimensioned waveguide section (which comprises the nanotaper), forming a “mode conversion region”. Referring now to FIG. 1, reference numeral 1 denotes a single mode waveguide, reference numeral 2 denotes a mode field size conversion region, reference numeral 3 denotes an auxiliary waveguide section, reference numeral 4 denotes a nanotaper, and reference numeral 7 denotes a low index auxiliary waveguide. FIG. 2 best illustrates the geometry of nanotaper 4 along the surface of the optical substrate. Within mode field size conversion region 2, nanotaper 4 has a width that starts at a relatively small value at tip 5 (often 50-150 nm), and then tapers outward to the final desired waveguide dimensions associated with single mode optical waveguide 1. The thickness x of nanotaper 4 remains relatively constant along mode field size conversion region 2, where thickness x is best shown in FIG. 1.
The mode size associated with tip 5 of the nanotaper 4 is “large” (due to the weak confinement of the light) and shrinks as nanotaper 4 expands in size, providing tighter confinement of the light as the effective refractive index increases along the length of the nanotaper. This effect facilitates the required mode conversion into the smaller mode associated with ultrathin single mode waveguide 1.
In use, light is launched into an endface 6 of auxiliary waveguide section 3, where it propagates along unimpeded until it encounters tip 5 of nanotaper 4 in mode conversion region 2. At this point, the light beam is transferred from the relatively low effective index layer 7 of auxiliary waveguide section 3 to the relatively high effective index ultrathin waveguide 1 with low loss, since the mode size is gradually reduced along the extent of the taper.
Even when using such an auxiliary waveguide, coupling loss occurs as a result of mis-alignment between the incoming optical signal and the auxiliary waveguide. The configuration of the auxiliary waveguide also contributes to signal loss, associated with the incomplete mode conversion between the auxiliary waveguide and the nanotaper.